Conventionally, as shown in FIG. 11, an electric power tool A includes a main body 201 and a rechargeable battery pack 202 detachably attached to the main body 201. As a mechanism for attaching and detaching the battery pack 201 to and from the main body 201, there has been employed, e.g., a structure, in which a pair of locking pieces 233 provided in symmetrical positions on the left and right sides of the battery pack 202 are independently engaged with a pair of engaging recesses 236 of the main body 201. More specifically, each of the locking pieces 233 is provided, at one end, with a hook portion 234 which is biased by a spring 235 into engagement with the corresponding engaging recess 236 and, at the other end, with a release operation portion 206 which may be pressed inwardly against the biasing force of the spring 235 to thereby release the hook portion 234 from the corresponding engaging recess 236 (see, e.g., Japanese Patent Laid-open Application No. 2006-263830).
The attaching and detaching mechanism is designed to ensure that, even when the electric power tool A is bumped against other objects and eventually the one release operation portion 206 of one of the locking pieces 233 is erroneously pressed, the other hook portion 234 of the other locking piece 233 remains engaged with the corresponding engaging recess 236. Thus the battery pack 2 is prevented from being dropped from the main body 201. However, in case the battery pack 202 is hastily grasped and the release operation portions 206 of the pair of locking pieces 233 are simultaneously pressed erroneously, the battery pack 202 may be dropped from the main body 201.
As noted above, in view of the ease of detachment, the attaching and detaching mechanism ensures that the battery pack 202 can be detached from the main body 201 by using the pair of the graspingly operable release operation portions 206 of the same kind arranged in the left and right symmetrical positions. For this reason, there still exists a high risk that the battery pack 202 is dropped from the main body 201 by the erroneous grasping operation to press the release operation portions 206 at the time when the electric power tool A is hastily grasped. That is to say, the attaching and detaching mechanism has no capability of reliably preventing the battery pack 202 from being inadvertently separated from the main body 201.